1. Field of the Invention
The present invention relates to a method for supporting wireless connectivity of a mobile station to a cellular network, wherein said mobile station is located within the coverage area of a femtocell base station of which it is a non-CSG mobile station, and wherein said mobile station experiences interference which it indicates to said femtocell base station.
Furthermore, the present invention relates to a femtocell base station for supporting wireless connectivity of mobile stations to a cellular network, being configured to receive interference indications experienced by non-CSG mobile stations that are located within its coverage area.
2. Description of the Related Art
In recent years there is an interest from mobile network operators to deploy femtocells (also known as home base stations, home BTS, home NBs, femto access points (FAPs), or femto radio base stations) which can be installed within the homes of the operators' customers. Such femtocells, currently being developed and standardized for both 3G and 4G networks, are scaled-down low-cost mobile base stations (BS) with low transmit power. Installation of femtocell base stations will typically be handled by the customers themselves without any technical training, therefore it has to be a simple plug-and-play procedure. Femtocell base stations are similar to a WiFi Access Point, but the radio interface is based on wide area cellular network standards such as WiMAX (Worldwide Interoperability for Microwave Access), UMTS (Universal Mobile Telecommunications System) or 3GPP LTE (Long Term Evolution). Generally, femtocell base stations are connected to the operators' network via a virtual private network connection over a normal broadband internet connection—backhaul connection—like DSL or FTTH (Fiber to the home).
One reason for the introduction of femtocells is the increase of operator network coverage and throughput for the sake of a better user experience and therewith to make a big step towards fixed-mobile-convergence. Like Wi-Fi access points, femtocells are designed to be deployed in home and office environments in order to give full coverage in the respective area and deployment of high density is expected in areas with high population or office density. However, the deployment of femtocells comes along with drawbacks of which one is interference effects between macro and femto levels of the network. Since femtocell base stations are often deployed in the same licensed spectrum as the wide area network of the operator, it is important to configure the radio interface correctly to avoid interference with other base stations.
Femtocell networks can improve indoor coverage and capacity. However, a femtocell base station needs to carefully balance its transmit power, which should be high enough to ensure good signal strengths for its subscribers, but not too high to avoid severe interference to other mobile stations, other femtocells and possible co-located macrocell base stations. To address this problem, according to prior art solutions, femtocells are linked for example to the network's Radio Resource Management (RRM), which is a functional/logical unit that is typically located in the Radio Access Network and that is responsible e.g. for frequency allocation and controlling the transmit power.
According to the definition in the current standard IEEE P802.16m, in terms of their subscription characteristics femtocell base stations are categorized into three main categories of subscriber types. The main categories are Open Subscriber Group (OSG), Closed Subscriber Group—Closed (CSG-Closed) and Closed Subscriber Group—Open (CSG-Open). These categories can be described as follows:                An Open Subscriber Group (OSG) femtocell base station is accessible to any mobile station (MS).        In CSG-Closed, the femtocell base station is accessible only to mobile stations which are members of this femtocell base station except for emergency services.        A CSG-Open femtocell base station is primarily accessible to the mobile stations that belong to its Closed Subscriber Group (CSG), while other mobile stations outside the CSG—i.e. non-member mobile stations—may also access such femtocell base station, but the non-member mobile stations will be served at lower priority. The CSG-Open femtocell base station will only provide service to non-member mobile stations as long as the Quality of Service (QoS) of mobile stations in its CSG is not compromised. That means best-effort services for non-CSG mobile stations of this femtocell base station.        
Generally, the owner of the backhaul connection and/or the femtocell base station may want to restrict access to the femtocell base station because he pays the expense of maintenance and the broadband connection to his premises. Thus, the categorization according to the standard IEEE P802.16m causes the problem that when an MS, not being a member of the nearby CSG-Closed femtocells, enters an area which is covered only by one or more CSG-Closed femtocell base stations, the communication of the MS with its macrocell base station may be severely interfered or even totally interrupted. Specifically, for example if a Non-CSG member MS, which is not a member of the one or more nearby CSG-Closed femtocells, enters a CSG-Closed only area or is switched on in a CSG-Closed only coverage area, the interference from the surrounding femtocell base stations may interrupt the communication between the Non-CSG member MS and its serving (macro) base station or the nearest (macro) base station.
To solve the above problem, it has already been agreed within the current IEEE 802.16m standard to the following:
“If an advanced mobile station (AMS) is placed into outage by a CSG-closed Femto advanced base station (ABS) of which it is not a member, it can indicate this problem to that Femto ABS by sending an AAI_RNG-REQ with the “Femto Interference” bit set to 1.”
This would allow the non-CSG-closed MS to inform the femtocell base station as being the source of the interference to the MS.
Based on this, several solutions are proposed to mitigate the interference. The most popular solutions are as follow:
Upon communication with the network entity, i.e. upon sending the interference indication to the femtocell, the CSG-Closed Femto BS may convert to a CSG-Open Femto BS. Alternatively, it has been proposed that the CSG-Closed Femto BS reduces its transmit power as directed by the network entity and/or that the CSG-Closed Femto BS refrains from transmitting on certain resource regions as indicated by the network entity.
However, all of these proposals are incomplete solutions, and typical questions such as the following still remain to be addressed:    1. How can the CSG-femtocell trust the MS, which sent the AAI_RNG-REQ, to be a legal MS?    2. How can the CSG-femtocell be convinced that it is the source of interference if there are other surrounding Base Stations around?    3. What is the interference level?